In silico strategy to design an efficient organic photoswitch based on excited-state cation transfer.

A new class of photoswitches and the corresponding elementary photoinduced reaction, the so-called Excited-State Cation Transfer (ESCT), are investigated. This reaction relies on an intramolecular photo-release/photo-complexation of cation: after irradiation, the cation is translocated from a complexation site 1 to a site 2 during the excited state lifetime. Our purpose is thus to develop a computational strategy based on Density Functional theory (DFT) and its time-dependent counterpart (TD-DFT) to improve the different properties of the ESCT photoswitches, namely (i) the ground state complexation constant K, (ii) the excited state complexation constant K*, (iii) the photoejection properties and (iv) the population of the triplet states from a singlet state via intersystem crossing to increase the lifetime of the excited state. In this work, we are interested in optimizing the ESCT properties of a betaine pyridinium chromophore substituted by a 15-aza-5-crown, that was previously shown to efficiently photoeject a Ca2+ cation from the site 1 but no photo-recapture was observed in the site 2 [Aloïse et al., Phys. Chem. Chem. Phys., 2016, 22, 15384]. To this purpose, we have investigated the impact of the modification of the site 1 on the ESCT properties by introducing different substituents (EDG groups, halogen atoms) at different positions. So far, promising systems have been identified but a simultaneous improvement of all the ESCT photoswitches properties has yet not been achieved.

Aggregation-Induced Emission and Circularly Polarized Luminescence Duality in Tetracationic Binaphthyl-Based Cyclophanes.

Here, we report an approach to the synthesis of highly charged enantiopure cyclophanes by the insertion of axially chiral enantiomeric binaphthyl fluorophores into the constitutions of pyridinium-based macrocycles. Remarkably, these fluorescent tetracationic cyclophanes exhibit a significant AIE compared to their neutral optically active binaphthyl precursors. A combination of theoretical calculations and time-resolved spectroscopy reveal that the AIE originates from limited torsional vibrations associated with the axes of chirality present in the chiral enantiomeric binaphthyl units and the fine-tuning of their electronic landscape when incorporated within the cyclophane structure. Furthermore, these highly charged enantiopure cyclophanes display CPL responses both in solution and in the aggregated state. This unique duality of AIE and CPL in these tetracationic cyclophanes is destined to be of major importance in future development of photonic devices and bio-applications.

Aggregation-Induced Emission: A Challenge for Computational Chemistry Taking TPA-BMO as an Example*.

A multi-environment computational approach is proposed to study the modulation of the emission behavior of the triphenylamine (Z)-4-benzylidene-2-methyloxazol-5(4H)-one (TPA-BMO) molecule [Tang et al., J. Phys. Chem. C 119, 21875 (2015)]. We aim at (1) proposing a realistic description of the molecule in several environments (solution, aggregate, polymer matrix), (2) modelling its absorption and emission properties, and (3) providing a qualitative understanding of the experimental observations by highlighting the photophysical phenomena leading to the emission modulation. To this purpose, we rely on (TD-)DFT calculations and classical Molecular Dynamics simulations, but also on the hybrid ONIOM QM/QM' approach and the in situ chemical polymerization methodology. In low-polar solvents, the investigation of the potential energy surfaces and the modulation of the emission quantum yield can be attributed to possible photophysical energy dissipation caused by low-frequency vibrational modes. In the aggregate and in the polymer matrix, the emission modulation can be qualitatively interpreted in terms of the possible restriction of the intramolecular vibrations. For these two systems, our study highlights that a careful modelling of the environment is far from trivial but is fundamental to model the optical properties of the fluorophore.

Understanding the properties of dithienylethenes functionalized for supramolecular self-assembly: a molecular modeling study.

A dithienylethene (DTE) photochromic compound functionalized by ureidopyrimidinone (UPy) quadruple hydrogen bonding blocks was synthesized by Takeshita and coworkers [Takeshita et al., Chem. Commun., 2005, 761] in order to form a light-responsive supramolecular self-assembling system. In solution, the formation of supramolecular assemblies was only observed for one DTE isomer, namely the closed-form isomer. To rationalize this experimental finding, with the help of Molecular Dynamics (MD) and (time-dependent) DFT calculations, the behaviour of open-form and closed-form monomers, dimers, hexamers and π-stacked dimers in solution is investigated. Our simulations show that, for the open-form oligomers, the progression of the supramolecular assembly is hindered due to (i) the possible formation of a very stable cyclic dimer for the open-form parallel isomer, (ii) the relative flexibility of the open-form oligomers compared to their closed-form counterparts, and (iii) the possible existence of π-stacked dimers that constitute bottlenecks blocking the progression of the supramolecular self-assembly.

Unraveling ultrafast dynamics of the photoswitchable bridged dithienylethenes under structural constraints.

The excited state dynamics of constrained photochromic benzodithienylethenes were addressed by considering the bridging with polyether chains (from x = 4 to 6 units) at the ortho and meta positions of the aryl group, named DTE-ox and DTE-mx, via time-resolved absorption spectroscopy supported with (TD)-DFT calculations. The photochromic parameters and geometrical structures of these series are discussed. A novel photocyclization pathway via a triplet state, evidenced recently (Hamdi et al., Phys. Chem. Chem. Phys., 2016, 18, 28091-28100), is largely dependent on the length and the position of the polyether chain. For the first time, by comparing the two series, we revealed, for the DTE-ox series, an interconversion not only in the ground state but also between the triplet states of the anti-parallel and parallel open form conformers.

Aggregation-caused quenching versus crystallization induced emission in thiazolo[5,4-b]thieno[3,2-e]pyridine (TTP) derivatives: theoretical insights.

We report a QM (TD-DFT) and QM/QM' (ONIOM) study of the modulation of emission in a series of thiazolo[5,4,b]thieno[3,2-e]pyridine (TTP) derivatives [Huang et al., J. Mater. Chem. C, 2017, 14, 3456]. By computing the excitation energy transfer couplings and the Huang-Rhys (HR) factors, we rationalize the aggregation-caused quenching (ACQ) observed for the parent molecule and the crystallization-induced emission (CIE) observed for the derivatives presenting intra-molecular H-bonding. We also show that the CIE strategy relying on the rigidification of the arch-bridge-like stator should be considered with caution since it can promote the energy dissipation through vibrational motions.

Modeling the photosensitizing properties of thiolate-protected gold nanoclusters.

An accurate computational strategy for studying the structural, redox and optical properties of thiolated gold nanoclusters (GNCs) using (Time-Dependent) Density Functional Theory is proposed. The influence of the pseudopotential/basis set, solvent description and the choice of the functional has been investigated to model the structural and electronic properties of the Au25(SR)18(-) system, with R being an organic ligand. This study aims to describe with a comparable precision both the GNC and the organic ligands and rationalize the effect of coating on different GNC properties. Two differently coated GNCs have been considered: the system with R = CH2CH2Ph and the GNC coated with 17 alkyl chains (C6H13) and functionalized by one fluorophore pyrene derivative (CH2CH2(NH)(CO)Py). The computational protocol we propose should then be used to design more efficient metal cluster-sensitized solar cells.

Interaction of osmium(ii) redox probes with DNA: insights from theory.

In the course of developing ultrasensitive and quantitative electrochemical point-of-care analytical tools for genetic detection of infectious diseases, osmium(ii) metallointercalators were revealed to be suitable and efficient redox probes to monitor the in vitro DNA amplification [Defever etal, Anal. Chem., 2011, 83, 1815-1821]. In this work, we thus propose a complete computational protocol in order to evaluate the affinity between Os(ii) complexes with double-stranded DNA. This protocol is based on molecular dynamics, with the parametrization of the GAFF force field for the Os(ii) complexes presenting an octahedral environment with polypyridine ligands, and QM/QM' calculations to evaluate the binding energy. For three Os(ii) probes and different binding sites, molecular dynamics simulations and interaction energies calculated at the QM/QM' level are successively discussed and compared to experimental data in order to identify the most stable binding sites. The computational protocol we propose should then be used to design more efficient Os(ii) metallointercalators.

Accidental degeneracy in the spiropyran radical cation: charge transfer between two orthogonal rings inducing ultra-efficient reactivity.

Photochromism of the spiropyran radical cation to the corresponding merocyanine form is investigated by a combination of electrochemical oxidation, UV/vis absorption spectroscopy, spectroelectrochemistry and first-principles calculations (TD-DFT, CAS-SCF and CAS-PT2). First, we demonstrate that the ring-opening of mono-spiropyrans occurs upon one-electron oxidation and that it can be driven photochemically as well as thermally, with trapping of the merocyanine by protonation. Second, in order to explain this experimentally observed spectroelectrochemical behaviour we suggest a theoretical mechanism based on the reactivity of the two lowest electronic excited-states, which promotes effective electron transfer from the indoline (nitrogen-ring) to the pyran (oxygen-ring) moieties (and vice versa) through a conical intersection seam of degeneracy. Characterisation of the minimum energy conical intersection on this crossing revealed that it presents a rare diabatic trapping topology. The excited state molecule cannot escape from crossing the intersection seam due to the presence of only one degeneracy-lifting coordinate that efficiently channels into the formation of the merocyanine photoproduct, so giving rise to a "kitchen sink" funnel-like effect. Therefore, assuming rapid relaxation after vertical excitation to a higher electronic state, photoconversion cannot be avoided in the D1 electronic state, which rationalises the remarkably efficient visible light driven excited-state reactivity observed experimentally.

Multiphotochromic molecular systems.

Molecular systems encompassing more than one photochromic entity can be used to build highly functional materials, thanks to their potential multi-addressability and/or multi-response properties. Over the last decade, the synthesis and spectroscopic and kinetic characterisation as well as the modeling of a wide range of multiphotochromes have been achieved in a field that is emerging as a distinct branch of photochemistry. In this review, we provide an overview of the available multiphotochromic compounds which use a variety of photoactive building blocks, e.g., diarylethene, azobenzene, spiropyran, naphthopyran or fulgimide derivatives. Their efficiency in terms of multi-responsiveness is discussed and several strategies to circumvent the most common limitation (i.e., the loss of photochromism of one part) are described.

SCC-DFTB parameters for simulating hybrid gold-thiolates compounds.

We present a parametrization of a self-consistent charge density functional-based tight-binding scheme (SCC-DFTB) to describe gold-organic hybrid systems by adding new Au-X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold-thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and Aun SCH3 (n = 3 and 25) molecular-sized models. The geometrical, energetic, and electronic parameters obtained at the SCC-DFTB level for the small Au3 SCH3 gold-thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold-thiolate model, Au25 SCH3 , the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC-DFTB parameters are also in good agreement with DFT results.

Morphological and charge transport properties of amorphous and crystalline P3HT and PBTTT: insights from theory.

We explore the relation between the morphological and the charge transport properties of poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) semiconductor polymers in both amorphous and crystalline phases. Using molecular dynamics to simulate bulk supercells and the Marcus theory to analyze the transport properties we found that amorphous systems display a reduced hole mobility due to the loss of nematic order and π-π stacking leading to a reduction in the electronic coupling between two chains. In the crystal phase, PBTTT displays a larger charge mobility than P3HT due to the interdigitation of the side chains enhancing the stability of the conjugated rings on the backbones. This more stable π-π stacking reduces the energetic disorder with respect to P3HT and increases the electronic coupling. In contrast, in the amorphous phase, PBTTT displays a reduced charge mobility with respect to P3HT due to the absence of side chains attached to the thienothiophenes, which increases their fluctuations and the energetic disorder. In addition, we show that it is possible to calculate the reorganization energy neglecting the side chains of the polymers and thus saving computational time. Within this approximation, we obtained mobility values matching the experimental measurements, thus confirming that the side chains are crucial to shape the morphology of the polymeric systems but are not involved in the charge transport process.

A DFT study of a new class of gold nanocluster-photochrome multi-functional switches.

With the help of a computational scheme combining molecular dynamics, DFT and TD-DFT methods, the conformational, electronic and optical properties of a new class of hybrid compounds where a photochromic molecule belonging to the dithienylethene family (DTE) is covalently linked to a Au25 nanocluster (gold nanocluster or GNC) are investigated. We compare two types of hybrid GNC-DTE systems where the aromatic linker between the metallic and the DTE moieties is either a phenyl or a thiophene ring. By examining the perturbation of the DTE electronic structure after grafting upon the GNC, we show that the hybrid system with a phenyl linker should preserve its photochromic activity. For the latter system, we have then studied the possible energy and electron transfer between the GNC and the DTE units. The energy transfer between the two moieties can be a priori discarded while a uni-directional electron transfer should take place from the GNC to the excited DTE. We show that this transfer can be controlled by switching the state of the molecule.

The photochemistry of inverse dithienylethene switches understood.

The photophysical properties of a series of dithienylethenes, free or blocked in an ideal photoactive conformation by an alkyl bridge, have been investigated by stationary, ultrafast spectroscopy and state-of-the-art time-dependent density functional theory calculations. Thanks to the clear ultrafast transient signatures corroborating NMR results, we bring strong evidence that the unreactive parallel open form conformer has been efficiently removed by the chain. For the first time, the photophysics of this species, namely an internal conversion of 120 ps is highlighted. In contradiction to the main ideas in the literature, the photocyclization mechanism is rationalized by a direct photocyclization mechanism from the Franck-Condon region passing directly through a conical intersection within ≈100 fs (not few picoseconds) while a competitive mechanism occurs through the relaxed S1 state. Relaxation processes (fluorescence and internal conversion) originating from this relaxed state are sensitive to the length of the blocking chain. Both concomitant pathways are necessary to rationalize: (i) the inverse relationship between emission and cyclization quantum yields and (ii) the non-unity value of the latter for bridged compounds.

New insights into the by-product fatigue mechanism of the photo-induced ring-opening in diarylethenes.

The photochromic properties of diarylethenes, some of the most studied class of molecular switches, are known to be controlled by non-adiabatic decay at a conical intersection seam. Nevertheless, as their fatigue-reaction mechanism - leading to non-photochromic products - is yet to be understood, we investigate the photo-chemical formation of the so-called by-product isomer using three complementary computational methods (MMVB, CASSCF and CASPT2) on three model systems of increasing complexity. We show that for the ring-opening reaction a transition state on S1(2A) involving bond breaking of the penta-ring leads to a low energy S1(2A)/S0(1A) conical intersection seam, which lies above one of the transition states leading to the by-product isomer on the ground state. Therefore, radiationless decay and subsequent side-product formation can take place explaining the photo-degradation responsible for the by-product generation in diarylethene-type molecules. The effect of dynamic electron correlation and the possible role of inter-system crossing along the penta-ring opening coordinate are discussed as well.

Competitive direct vs. indirect photochromism dynamics of constrained inverse dithienylethene molecules.

State-of-the-art experimental and theoretical tools were used to investigate the gas-phase relaxation dynamics of various photoexcited photochromic dithienylethene molecules in situations where several relaxation channels are simultaneously at play. Unconstrained and constrained dynamics were addressed by considering unbridged and bridged molecules with a polyether bridge of various sizes (from 2 to 4 units). Time-resolved ultrafast ionization spectroscopy techniques were used to probe the dynamics. This revealed the existence of several relaxation pathways from the first excited state to the ground-state. Characteristic times were determined for each process. These channels compete at an early stage of the dynamics only when the initial wavepacket splits into two parts. A striking excited state wavepacket oscillation is observed in bridged molecules. A general reaction mechanism is proposed which rationalizes the carbon-carbon distance rule which is widely used as an empirical tool to predict the photoactivity of photochromic molecules in crystals.

Excited-state dynamics of thiophene substituted betaine pyridinium compounds.

This work deals with the photophysics of novel pyridinium betaine based on 2-pyridin-1-yl-1H-benzimidazole (SBPa) substituted symmetrically by mono- (Th2SBPa) and bi-thiophene fragments (Th4SBPa). The study is based on a combination of steady-state, femtosecond transient absorption spectroscopic measurements supported by PCM-(TD)DFT calculations. It is found that the two step ICT process (S0 → S2 excitation followed by S2(CT) → S1(CT) internal conversion) occurring for the parent molecule remains unaffected for Th2SBPa while the situation is less clear for Th4SBPa. Actually, for both molecules, a new decay route involving the π-electron system localized in thiophenic groups is responsible for the production of triplet states. Involvement of this new route in the parallel production of S1(CT) is strongly suspected.

Density functional theory study of the conformation and optical properties of hybrid Au(n)-dithienylethene systems (n = 3, 19, 25).

We present a theoretical study of Aun-dithienylethene hybrid systems (n = 3, 19, 25), where the organic molecule is covalently linked to a nanometer-scaled gold nanoparticle (NP). We aim at gaining insights on the optical properties of such photochromic devices and proposing a size-limited gold aggregate model able to recover the optical properties of the experimental system. We thus present a DFT-based calculation scheme to model the ground-state (conformation, energetic parameters) and excited-state properties (UV-visible absorption spectra) of this type of hybrid systems. Within this framework, the structural parameters (adsorption site, orientation, and internal structure of the photochrome) are found to be slightly dependent on the size/shape of the gold aggregate. The influence of the gold fragment on the optical properties of the resulting hybrid system is then discussed with the help of TD-DFT combined with an analysis of the virtual orbitals involved in the photochromic transitions. We show that, for the open hybrid isomer, the number of gold atoms is the key parameter to recover the photoactive properties that are experimentally observed. On the contrary, for hybrid closed systems, the three-dimensional structure of the metallic aggregate is of high impact. We thus conclude that Au25 corresponds to the most appropriate fragment to model nanometer-sized NP-DTE hybrid device.

Single molecule multiphotochromism with diarylethenes.

In single photochromes, the two isomers that are interconverted in photoinduced reactions can serve as on and off states in a molecular switching device. The addition of several photochromic moieties onto a single molecule can allow the processing of more complex logical patterns. For example, an asymmetric triad could, in principle, store a byte, rather than a bit, of data. Because of the potential impact of multiphotochromic molecules in many research areas, over the past decade several groups have synthesized these coupled structures. The targets are easily addressable molecules that display increased contrast between the on and off states and in which all isomers have significantly distinguishable optical signatures. In this Account, we provide an overview of the multiswitchable molecular systems that incorporate at least one diarylethene group, which is the most successful thermally stable (P-type) organic photochrome. Up to this point, most systems have presented significant limitations. First of all, the reversibility of the processes is hindered by several side reactions more frequently than for single photochromes. Second, switching one part of the compound impedes the photoreactivity of other fragments in approximately 50% of the cases, and maximizing the electronic communication increases the probability of partial activity. In addition, most of the few synthesized operative systems only demonstrate cumulative absorption spectra rather than new features. Finally, it is impossible to selectively induce a chosen conversion because one wavelength might trigger several processes. We also emphasize the promising successes of asymmetric diarylethene dimers and trimers and molecules that combine two families of photochromes, such as diarylethene added to fulgimide or phenoxy-naphthacenequinone. In that framework, theoretical simulations offer complementary tools to investigate these structures, both to obtain structure/property relationships and to propose paths for the design of more efficient molecules. However, due to the size of the systems, researchers can only apply semiquantitative models. The investigation of the absorption spectra of the photochromes with time-dependent density functional theory (TD-DFT), the analysis of the topology of the LUMO + n (typically n = 1) of the closed-open hybrid, and an estimate of the steric stress in the hypothetical (ground-state) closed-closed structure serve as a useful combination of parameters to obtain initial insights regarding the photocyclization of the different open diarylethene groups. Nevertheless, because a first-order qualitative approach does not explore the potential energy surface of the photoexcited states, it remains inadequate for the investigation of some molecules.

Do inverse dithienylethenes behave as normal ones? A joint spectroscopic and theoretical investigation.

We investigate an inverse (I) dithienylethene, the bis(3,5-dimethyl-2-thienyl) perfluorocyclopentene, using absorption, emission and NMR spectroscopies as well as state-of-art first-principles (TDDFT) calculations. First, we find in addition to the expected antiparallel and parallel conformers, a new stable antiparallel conformer , but its energy is too high to be significantly populated at working temperature. More importantly, we demonstrate that, instead of an equal proportion of an AP and a P conformer as in normal (N) diarylethenes, the AP conformer is present in large excess. This result is confirmed by both DFT thermodynamical analysis and temperature-dependent NMR experiments modelized with an ↔ fast interconversion model. With the latter, the relative populations are estimated to be ca. 3/1 for /. Furthermore, the 0-0 energies simulated with a model that accounts for both vibrational and state-specific media effects of the ground and the excited states indicate that and have very similar absorption signatures while only the conformer should give rise to emission. Eventually, within excited state manifold, important topological points along the ring-closure reaction coordinate, and more specifically the unprecedented S1(opt) of the closed isomer, have been identified.